Amplifier circuit



Nov, 22, 1938.

F.H.SHEPARD.JR

AMPLIFIER CIRCUIT VFiled. May 28, 1934` 2 Sheets-Sheet 1 FRANCIS H.5HEPARD JR.

BY Q

)fg Mm TTORN Ev Nov. 22,1938. F. H. sHIEPARD. JR 'y 2,137,419

AMPLIFIER CIRCUIT 5 C ,im j kfz 6/ 0f INVENTOR FRANCIS H. SHEPARD JR.`

` TTORN w Patented Nov. 22, 1938 PATENT oFFicE ADIPLIFIER CIRCUIT Francis H. Shepard, Jr., Rutherford, N. J., as#

signor to Radio Corporation of America, a corporation of Delaware Application May 28, 1934, Serial No. 727,968

10 Claims.

The present invention relates generally to amplifiers and more particularly to so called D. C. ampliers, of the type adapted to amplify D. C. and low frequency A. C.

For 'many applications it is desirable to have an amplifier that will efciently amplify small D. C. voltages. In systems heretofore known, where more than one stage of amplification is needed, D. C. ampliiication is not obtained easily for the reasons that either there must be separate batteries for each stage, or a set of bucking batteries for each stage, or a set of divider resistors foreach stage, or again, a very high voltage source across which the various stages m-ay be placed in series with each other, or a modulator system where the D. C. is converted into A. C., amplified and then re-converted back into D. C. All of the methods mentioned above are more or less complicated and furthermore are quite critical to adjust.` It is also desirable to have an amplifier oper-ating directly on raw A. C. for use with a photo tube and other industrial uses as for instance temperature control systems and burglar alarm systems.

The present invention may be broadly stated to comprise a method and means for cascading grid controlled rectiiers to produce a D. C. amplifier system which does not necessitate cascading the necessary tube energizing voltages. The invention also contemplates a method and means for correcting for variations in the supply voltage.

Bro-adly speaking, in the present system the negative voltage developed on the plate of a grid controlled rectier tube connected across an A. C. line is used to bias the grid of a following tube alsoconnected across the line. The energy to be amplified is impressed -across the input of the irst tubeA and the amplified energy is available across the output of the last tube.

The various broad aspects of the invention are illustrated by the circuit diagrams in the accompanying drawings wherein,

Figures 1-4 represent curve sheets showing various tube operating characteristics of use in describing the present invention;

Figure 5 illustrates diagrammatically a circuit diagram incorporating, in a simple manner, the broad features of the present invention;

Figure 6 is a diagrammatic representation of a circuit arr-angement differing. from that shown in Figure 5 only in that means are provided for increasing the gain of the amplifier by causing the tubes to be operated at or near the steep part of their grid voltage plate current curves;

Figure 7 shows an improved circuit arrangement over the system shown in Figure 6 eliminat, ing the use of additional voltage sources;

Figure 8 illustrates a system like that shown in Figure '7 except that means are provided for eliminating the eiiects of line voltage variations; and,

Figure 9 is a diagrammatic representation of an improved and simplified ampliiier incorporating the features of the present invention.

Attention is now directed to Figure 5 wherein lines 5 and 6 are adapted tobe connected to a suitable commercial A. C. power supply network .by means of terminals I and 2. A space discharge device T1 provided with anode, cathode and grid electrodes acts as a first amplifier for the D. C. input. 'I'he input circuit of tube T1 comprises a connection between the grid electrode and the cathode and includes terminals 3 and 4, across which the D. C. to be amplified may be applied in any desired manner. A bias battery B1 may be provided in the input circuit so as to maintain the grid of the tube T1 at a predetermined operating potential with respect to the cathode thereof. The cathode of tube T1 is also connected to the line 6, while the anode of the tube is connected to the line 5 through a resistor R1 which is shunted by a condenser C1. The anode of tube T1 is also connected to the line 6 through a resistor l'tz` and a series condenser C2. A second amplification stage may be provided and comprises an electronic tube T2 having an anode, a cathode and a control grid. 'I'he grid of the tube T2 is connected directly to a point intermediate resistance R2 and condenser C2 and there is thus provided a. direct connection between the anode of tube T1 and the grid of tube T2 through resistor R2. The anode of tube T2 is connected to the line 5 through a suitable resistor Rs which is shunted by a condenser Ca while the cathode of the tube is connected directly to line 6. The anode of tube T2 is also connected to line 6 by means of a connection including resistor R4 and a series condenser C4.

A third stage may be similarly connected. This stage may also include an electronic tube T3 provided with anode, cathode and grid electrodes. The cathode of T3 is shown connected to the line 6. The anode is connected to the line 5 through a resisto-r R5 shunted by a condenser C5. The control grid is connected to a point intermediate resistor R4 and condenser C4. It is obvious that any number of succeeding stages may be added, it being understood that three stages have been shown for purposes of illustration only.

In Figure 5 a utilizing circuit or load shown generally as a voltmeter V is connected across the resistor R5.

From the above it can be seen that the circuit shown in Figure 5 cascades three tubes which have A. C. on their plates, for the purpose of amplifying D. C. and/or low frequency A. C. voltages applied to the input terminals 3 and 4. In operation, the rectifying action of each tube causes a'pulsating D. C. current to flow through the tube so that the D. C. potential of its plate or anode becomes negative with respect to the line. The grid bias on a tube will determine its effective plate conductance while conducting, that is, during the half cycle when the plate is positive, and hence, will control the amount of rectified current passed through its respective plate resistor. The IR drop variations across the plate resistor are in excess `of the bias variations, hence in accordance Ywith Well known principles, the tube Will act as an amplifier. As the D. C. potential across both sides of an A. C. line is zero, the voltage drop across the plate resistor of any tube may be used after passing it through a suitable lter so as to obtain an average value, as bias for the next tube. In this way any number of stages may be used to obtain the required'voltage amplification. In practice, it has been found that using type 53 tubes in a circuit similar to that shown in Figure 5, a voltage gain of about Il per stage can be readily realized.

In a circuit such 'as shown in Figure 5 the gain of any tube is limited by the factthat the D. C. voltage drop in its respective plate resistor is limited to the grid bias voltage required for the next tube. This makes it necessary'to use a very low value of plate resistor or to operate the tubes with a very high negative bias in order to make the grid bias equal the plate load voltage drop. In either of the above cases a great sacrifice in eilciency is made.

The circuit arrangement shown in Figure 6 is concerned with a system for overcoming the above mentioned loss in efliciency. It is apparent at once from a study of the circuit shown in Figure 6, that the only difference in the systems shown in Figures 5 and 6 is that in the latter a' battery B2 is placed in series with the A. C. line so that the drop in the plate resistor might be increased to a value equal to the battery voltage plus the grid bias voltage. This arrangement makes it possible to operate the tubes more efiiciently and, in fact, it was found in an actual set up using type 53 tubes that a voltage gain of between and 35 per stage may be easily attained. l

It is quite obvious from a consideration of the system disclosed in Figure 6 that the use of a separate source such as battery B2 is inconvenient and should be avoided.v For this purpose, the system shown in Figure 7 will now be considered. In Figure '7 the battery B2 of Figure 6 has been displaced by the diode rectifier D1 connected across the line 5, E and a condenser Ce, interposed between terminal l and the connection between the cathode of D1 and line 5. In

7o; connection with the systems shown in Figures 5 er or diode D1 such as shown in Figure '7 not only does away with the necessity for a battery such as B2, (Figure 6) but also acts to partly counteract the above mentioned instability due to line voltage variations. For instance, in the system shown in Figure 7, if the line voltage falls, the drop across the plate resistor of a tube will tend to become less with the result that the grid biases of the various tubes Will tend to decrease. However, the superimposed D. C. on the line will also decrease and this will tend to make the plate potentials or grid biases of the various tubes more negative.

Thus, the change in rectified superimposed D. C. due to the line voltage variations can be made to oppose the effects of the A. C. line voltage variations, if it is so desired. In an actual circuit built as shown in Figure 7 it was found that the instability for a given sensitivity can be reduced to approximately 1/6 of that obtained in circuits such as those described in Figures 5 and 6.

As pointed out above in connection with the description of the circuit arrangement shown in Figure '7 the instability in that system due to line voltage variations is only partially counteracted. It is conceivable that in some systems it would be highly desirable to have a circuit arrangement for the purposes described which within certain limitations is absolutely independent of line voltage variations. Such a circuit arrangement has been shown in Figure 8 wherein a potentiometer resistance R6 is connected across the lines 5 and 6 and a connection from the anode of tube T1 to a point on said resistor Re including a variable tap arrangement 'l and condenser C1 is provided. Condenser C1 in Figure 8 instead of directly shunting the resistor R1 as in Figure 7 is connected to the supply lines through the potentiometer resistor Re. In other respects the circuit arrangement of Figure 8 is similar to that shown in Figure 7. By means of the potentiometer 1, Re the A. C. voltage on the plate of tube T1 can be varied independently as regards the D. C. voltage: In this way the A. C. and D. C. effects described above in connection with Figure 7 can be made to perfectly compensate for each other and make the amplification and level of output current independent of line voltage va riations Within said limits. The theory underlying the above can best be illustrated by the q The value of this voltage can be controlled by the grid potential of the tube. When pure D. C. is applied to the plate of a vacuum tube through a load resistor the assumed D. C. potential of the plate is always positive and varies with the supply voltage. By placing both A. C. and D. C. on the plate of the tube simultaneously and keeping the ratio of A. C. to D. C. constant as is the case where the A. C. is rectified to supply the D. C. the two effects can be made to oppose each other for the effects of line voltage variations. If the proper AC/DC ratio is chosen there can be obtained practically perfect compensation over certain ranges of line voltage variations and grid bias variations. If the AC/DC ratio is too high over-compensation will result and if it is too low there is obtained under-compensation. Because of the fact that a single stage of the amplilier may be considerably over-compensated all the compensation for several stages in cascade may be obtained in one of the stages only, hence, as shown in Figure 8 the compensating effect is obtained only in the rst stage and may be of such avalue as to compensate for all the succeeding stages of the amplifier.

`While no mention has been made of the effects of heater current variations which cause effective changes in the temperature of and hence 'in the contact potential between cathode and grid and changes in the velocity of emission of electrons from the cathode, which changes are equivalent to `actual changes in grid bias, it is. apparent that the AC/DC ratio may be adjusted so that this heater effect can also be compensated for. However, the compensation in the case previously discussed will take place immediately with line voltage variations while the effects due to cathode temperature will lag behind the voltage changes. If the time constant of the -response of the amplifier is greater than the heating time of the cathode, compensation can be accomplished `without any instantaneous decompensation.

For a clearer understanding of the operation of the system herein disclosed attenti-Onis now directed to Figures 1 through 4.

In Figure 1 there is shown a family of rectification curves for a triode with various values of A. C. on the plate and with Zero bias on the grid. Thisset of curves is similar to the curves usually shown for a diode detector. By means of the grid, the plate voltage current curve of a triode is shifted parallel to itself along the voltage axis, see Fig. 3. This causes the whole family of rectification curves to shift along the voltage axis.

Figure 2 shows how the rectification curves have shifted from the position shown in Figure 1 when thegrid biasis` changed from zero to a negative value. A load line A--B placed in Figures 1 and 2 will intersect the constant A. C. lines at different points on the two sets of curves. For instance, the load line in Figure 2 intersects the siX volt A. C. line at a plate potential of Zero volts.` With a less negative bias, as shown in Figure 1, the plate voltage will be some value less than Zero. In this way the grid bias can be used to control the potential on the plate and since this plate potential can be normally around zero it can be coupled (through a simple filter to remove the A. C.) directly to the grid of a following amplifier stage (see, for instance, Figure 8 where the anode of tube T1 is coupled directly to the grid of tube T2 through the resistor R2) the condenser C2 acting as a lter element.

Referring again to Figures 1 and 2, if the D. C. supply potential is increased the plate potential will increase. If the A. C. is increased the plate potential will decrease. If both the A.` C. and D. C. supplies are increased together inthe proper ratio there will be no change in the D. Cjplate potential. In the circuit arrangement shown in Figure 8 a rectifier is used to providethe D. C. supply, while the A. C. is supplied from the potentiometer arrangement 1, R5 connected across the line through a low impedance condenser C1 to the plate of the tube. The proper ratio between the A. C. and D. C. for perfect compensation may be obtained by adjusting the value of the A. C. by means of variable tap 'I.

As stated previously, it is possible to over-compensate or under-compensate the circuit so that the plate potential may vary in either direction with varying line voltage. Because of this fact it is possible to compensate in one stage the effects of line voltage variations in the other stages and it is also possible to compensate for the eiects of heater voltage variations, as will` be more fully covered below. As long as the plate current is small compared to the emission current of the cathode, that is, under unsaturated cathode current conditions, changes in the cathode temperature will have an effect, due to the velocity of emission of the electrons and contact potential on the cathode, equivalent to a change in grid bias.

Figure 4 shows a characteristic curve for bias which is equivalent to cathode exciter voltage changes. Since there is a definite relation between lament voltage and equivalent grid bias it is possible to compensate for filament voltage variations within limits in the plate circuit by the method outlined above.

Attention is now directed to Figure 9 which shows in diagrammatic form a simplied A. C. operated D. C. amplifier circuit that is self biased and compensated for line voltage variations. As in Figures 5 through 8 conductors 5 and 6 are connected to an A. C. power supply system through terminals I and 2. For purposes of illustration only three tubes T5, T5 and T7 have been shown comprising the amplifier, however, it is to be distinctly understood that any desired number of tubes may beutilized and connected as taught by the present disclosure. The rst tube T5 includes anode, cathode and grid electrodes. One side of the cathode is connected to line Ei while the anode is connected to conductor 5 through condenser C8. The anode is also connected to conductor t through two paths one of which includes the resistor Rin and the other a resistor R11 and a condenser C in series. The D. C. input is applied across terminals 3 and 4, terminal 3 being connected to the grid of T5 while terminal 4 is connected to a point on the resistor R10 by means of a variable tap I2. A condenser C7 is connected between the connection from terminal ll to the resistor R111 and conductor 6.

The second tube is also shown as including anode, cathode and grid electrodes, the anode being connected to the conductor 5 through a condenser C10 while one side of the cathode is connected directly to the conductor 6. The signal grid of the tube T5 is connected to a point on the resistor R11 by means of a variable tap I3. As in the case of tube T5 the anode of tube T5 is connected to the conductor E through two paths. One of these paths includes a resistor R12 while the other comprises resistor R13 and a condenser C11 in series. The third tube of the system T7 i-s also provided with anode, cathode and grid electrodes, the cathode being connected directly to the conductor E while the grid electrode is connected to a point intermediate resistor I3 and condenser C11. The anode of tube T1 is connected to an output terminal l I, terminal I0' shown at one end of the conductor 5 being the other output terminal of the system. It is to be understood that any desired utilizing system may be connected across the output ter- Ininals` I0 and II.

For a proper understanding of the operation of the system shown in Figure 9, attention is directed again to the curve sheets shown in Fig ures 1 and 2. It will be noted that the load line OC starts from Zero as there is no D. C. supply in the circuit shown in Figure 9. 'I'he plate voltage will always be negative as shown by the intersections with the triode rectification curves. The actual value of D. C. plate voltage with a constant A. C. voltage on the plate will vary with 'lil the grid bias. This is shown by observing the intersections of the constant A. C. lines with the D. C. load line in Figures 1 and 2. In normal operation, considering the tube T5, the anode will assume a highly negative average D. C. potential due to the rectifying action of the tube. This potential, for best amplification, will be too high a Value to use directly for self-bias. However, since the load resistor is returned to ground or zero potential, a slider may be placed on this resistor to obtain any average D. C. Voltage from zero to the average D. C. potential of the plate. It is possible in some particular instance that the D. .C. potential may be in the order of -80 volts.

If -4 volts isV the normal bias for the tube in this circuit, the slider is adjusted along the load resistor R10 (see Figure 9) until a potential of -4 volts is reached. Condenser C1 is sed as a filter to by-pass the A. C. on the top portion of the divider resistor R10. This will keep the A. C. oi the grid of the tube T5. If the normal bias is -4 volts, when the plate potential is -80 volts, the slider will be set approximately V20 of the way up the load resistor R10 and the degeneration due to the self-bias will be one part in 20, times the actual realized voltage gain. It should be understood, of course, that the values given above are approximate values that can be expectedV in normal operation of the system.

The potentiometer R11 as far as D. C. is concerned is merely a resistor of negligible Value in series with the grid of tube Ts. Condensers C10 and C11 have no eiect on the D. C. voltage, however, potentiometer R11 as far as A. C. is concerned will act as a voltage divider resistor across the line. Condensers Ca and Cs have negligible impedance to A. C. The instantaneous potential on the grid of tube Te is the sum of the D. C. voltage on the plate of tube T5 and the instantaneous value of A. C. at the point to which the slider I3 is set on the resistor R11. During the part of the cycle that the tube Te is conducting. the instantaneous A. C. voltage applied to the grid of tube Te is in such a direction as to oppose the normal D. C.,bias supplied by the plate of the vacuum tube T5. Thus, the grid during the conducting part of the cycle will be less negative than the normal bias and will be of such a value that the grid of the tube Ts will be at a suitable operating potential.

Suppose that the A. C. line voltage, that is, the A. C. voltage on the anode is increased it can be seen from the rectification curves in Figures 1 and 2 that the negative plate potential will increase. This will cause the normal grid bias of tube Ts to become more negative. However, increasing the A. C. line Voltage would also cause an increase in the superimposed A. C. on the grid on the tube T6 and this will tend to make the grid more positive during the operating part of the cycle. Since the two above mentioned effects can be made to oppose each other the eects of line voltage variations may be nullied. Compensation adjustment is made by varying the A. C. voltage on the grid of tube Ta by adjusting the slider I3 on the potentiometer R11. As previously stated, it is possible to obtain overcompensation, hence, it is possible to perform all the necessary compensation for several stages in the one stage. It is also possible as previously pointed out to accomplish compensation for the effects of filament voltage variations. The explanation for filament compensation in the circuit shown in Figure 9 is similar to that given above in connection with the description of curve sheet shown in Figure 4.

In the system shown in Figure 9 the slider I3 should be adjusted along the resistor R11 so that the grid of the tube Ts swings into the operating range of the tube but never swings positive with'respect to the cathode.

In practice it has been found that the values of none of the resistances and condensers used are critical. In one of a number of practical embodiments of the invention, and in particular one that followed the diagram shown in Fig. 8, a 2000 ohms resistor was used for the A. C. line shunt while the plate resistances were each one megohm, the grid resistors two megohms and the condensers 0.1 mmf.

While the invention has been described by certain particular embodiments, it is to be understood that the principles underlying the invention may be carried out in many other forms f put circuit for said tube including said grid electrode and the cathode, a capacitive impedance shunted across said resistor element and a filter circuit comprising an impedance device and a condenser connected between the anode of said tube and the cathode.

2. In a direct current amplifier, a pair of conductors adapted to be connected to a source of alternating current, a controllable rectifier cornprising an electronic tube having anode, cathode and control electrodes, a connection including a high plate resistor between the anode and one of said conductors, a connection between the cathode and the other conductor, an input circuit for said tube including the cathode and control electrode thereof and means adapted to connect the input circuit to a source of energy to be amplified, a second controllable rectifier comprising an electronic tube having anode, cathode and grid electrodes, a connection between the anode of the last named tube and one of said conductors including a plate resistor, a connection between the cathode of the last named tube and the other conductor, a direct connection between the anode of the iirst tube and the control electrode of the second tube including an impedance device, whereby a controlling voltage is impressed upon the control electrode of the second device as determined by the drop across the high plate resistor associated with the first tube due to the flow of current therethrough, said controlling voltage being normally too negative as regards the normal operating characteristics of the second tube to produce proper operation thereof and mea'ns for impressing a potential upon the anode of the rst tube with respect to the cathode thereof of such direction and amplitude to partially compensate for the high drop across the plate resistor of the first tube whereby there is impressed upon the control electrode of the second tube a control voltage commensurate with the proper operation of said tube. `1

3. In a direct current amplifier, a pair of conductors adaptedto be conneotedto a sourceof alternating current, a controllable rectifier comprising an electronic tube having anode, cathode and control electrodes, a connection including a high plate resistorbetween the anode and one of said conductors, a connection between the cathode and the other conductor, an input circuit for said tube including the cathode and control electrodethereof and means adapted to connect the input circuit to a source of energy to be amplied, a second controllable rectifier comprising an electronic tube having anode, cathode and grid electrodes, a connection between the anode of the last named tube and one of said conductors including a plate resistor, a connection between the cathode of the last named tube and the other conductor, a direct connection between the anode of the first tube and the control electrode of the second tube whereby a controlling voltage is impressed upon the control electrode of the second device determined by the drop across the high plate resistor associated with the first tube due to the flow of rectied current therethrough, said controlling voltage being normally too negative as regards the normal operating characteristics of the second tube to produce proper operation thereof and means for impressing a potential across said two conductors of such direction and amplitude as to partially compensate for the high drop across the plate resistor of the first tube whereby there is impressed upon the control electrode of the second tube a control voltage commensurate with the proper operation of said second tube.

4. The system described in the next preceding claim further characterized by that the means for impressing the potential across the two conductors comprises a rectifier device connected across the two conductors and arranged so that the positive potential side of the rectifier is connected to that conductor which is connected to the anode of the first tube.

5. The system described in claim 3 further characterized by that the means for impressing the potential across the two conductors comprises a rectifier device connected across the two conductors and arranged so that the positive potential side of the rectifier is connected to that conductor which is connected to the anode of the rst tube and wherein the rectier device is adapted to be connected to a source of alternating current through a condenser said system being further characterized by that the compensating voltage also acts to partly counteract instability of the amplifying system due to line voltage variations.

6. A method of amplifying a direct current voltage applied to a control grid of an electronic tube which comprises impressing both an alternating current voltage and a direct current Voltage upon the anode of the tube so that the tube will act as a rectifier and thereby build up a negative voltage on its plate, controlling the value of this negative voltage by the grid potential applied to the control grid and maintaining the ratio of the alternating current voltage to the direct current voltage applied to the plate constant and of such a value that effects of alternating current line voltage variations are i compensated. Y

between the alternating current and direct current voltage applied to the plate of the tube is maintained constant by deriving the direct current voltagefrom the alternating current voltage whereby variations in voltage of one causes `like voltage variations of the other.

8. In `anv alternatingcurrent operated direct currentv` amplifier, a source of l alternating current, a plurality of cascaded electronic rectifiers connected across the source in parallel, each of said rectifiers including a control element arranged so as to control the rectifying action of the rectifier, a source of energy to be amplified, means for connecting said last named source to the input of the first of said cascaded reotifi-ers to control its action in accordance with energy to be amplified, means for controlling the rectiiying action of each of the other rectifiers in accordance with the rectication of energy by the respective preceding rectifier, means for impressing a unidirectional potential across the space path of at least the first rectifier, the direction and amplitud-e of said potential acting to partially compensate for the potential applied to the next successive tube.

9. In a direct current amplifier, a source of alternating current including a pair of terminals, a first conductor connected to one of said terminals and a second conductor connected to the other of said terminals, a diode rectifier element including an anode and a cathode, the cathode of said rectifier element being connected to said first conductor and the anode thereof being connected to the other of said conductors, an elec'- tronic tube provided with an anode, a cathode and a control electrode, means including a plate resistor for connecting the anode to the rst named conductor, means for connecting the cathode to the other of said conductors, an input circuit for said electronic tube including the cathode and control electrode thereof, a connection between the anode of said electronic tube and the cathode thereof including a resistor and a capacity connected in the order named, a capacity element shunted across said plate resistor, a second electronic tube including an anode cathode and control electrode, a connection between the control electrode and the cathode thereof including said first named capacity element and means includinga plate resistor for connecting the anode of said second named electronic tube to the rst named conductor, a utilizing circuit and means for connecting the utilizing circuit across said last-named plate resistor.

10. In an amplifier circuit including at least two cascaded electron discharge devices each of said devices including anode, cathode and grid electrodes, a source of alternating current provided with a pair of terminals, a line conductor connected to one of said terminals and a line conductor connected to the other of said terminals, a resistor device connected between saidV two conductors, a diode rectifier having its cathode connected to the first of said conductors and its anode connected to the second, a connection between the anode of the first of said two electron discharge devices and the first of said conductors including a load resistance, a connection between the last named anode and a point of said first named resistor including a condenser, a connection between the other line conductor and the cathode of said first electron discharge device, an input circuit for said first electron discharge device including the grid and cathode thereof, a connection between the anode ofthe second of said electron discharge devices. and the iirst line conductor including a. load resistor shunted by a condenser, a connection between the anode of said second electron discharge device and the cathode thereof' including an impedance element,V means for connecting the cathode of the second electron discharge device and the cathode of the rst electron dischargei device, a connection between the anode of the first electron discharge device and the cathode thereof including a resistor and a condenser in series and a connection between the grid electrode of thev second named electron discharge device and a point of said last named circuit between the resistor and condenser.

FRANCIS H. SHEPARD, JR. 

